1. Field of the Invention
The present invention relates to receiver devices, error detection circuits, and receiving methods. More particularly, the present invention relates to a receiver device for receiving a signal that has been subjected to Orthogonal Frequency Division Multiplexing (OFDM), an error detection circuit for detecting an error between symbol timing of the received OFDM signal and internal symbol timing of the receiver device, and a method of receiving an OFDM signal.
2. Description of the Related Art
OFDM, which is one of multi-carrier transmission schemes, is tolerant to faults attributable to multipath delay and has begun to be used in terrestrial digital broadcasting and other applications.
FIG. 6 illustrates an OFDM signal.
A receiver device for receiving an OFDM signal carries out Fast Fourier Transform (FFT) with respect to a certain interval, or what is called effective symbol, in the OFDM signal in order to demodulate the received signal. Before each effective symbol, a signal called guard interval (GI) is inserted which is used for removing faults attributable to multipath delay. The guard interval and the succeeding effective symbol constitute one symbol of OFDM signal. The receiver device detects symbol timing from the received signal in the manner described below, by utilizing the fact that the signal inserted in the guard interval is a copy of the tail of the effective symbol.
FIG. 7 illustrates a method of detecting symbol timing.
The receiver device delays the received signal for one effective symbol period, then multiplies the received signal and the delayed signal together to obtain a correlation signal, and integrates the correlation signal by using its moving average. As a result, a peak appears at the symbol start position. Thus, the receiver device detects the symbol start position from the peak position, thereby capturing the symbol timing. This process is called symbol timing synchronization.
Following the symbol timing synchronization, the receiver device generates the next symbol start timing by counting the number of samples of the symbol by means of an internal clock, to determine FFT computation timing and the like.
Because of an error in the sampling frequency based on the internal clock of the receiver device, however, the symbol start timing generated within the receiver device occasionally deviates from the peak position of a moving average output signal shown in FIG. 7, causing a timing error.
In conventional receiver devices, the deviation is fed back as an error signal to control the sampling frequency so that the symbol start timing generated within the receiver device may always coincide with the peak position of the moving average output signal.
Also, there has been known a conventional method wherein the timing error is detected by obtaining a difference between areas of the moving average output signal corresponding to predetermined periods before and after the symbol start timing generated within the receiver device.
FIGS. 8A to 8C illustrate the method of detecting a timing error based on areas of the moving average output signal. FIG. 8A shows the case where the symbol start timing of the receiver device coincides with that of the received signal, FIG. 8B shows the case where the symbol start timing of the receiver device is earlier than that of the received signal, and FIG. 8C shows the case where the symbol start timing of the receiver device is later than that of the received signal.
Conventionally, a comparison is made between the areas of the moving average output signal corresponding to predetermined periods before and after the symbol start timing of the receiver device. In FIGS. 8A to 8C, SL5 indicates an area of the moving average output signal over the predetermined period before the symbol start timing generated within the receiver device, and SR5 indicates an area of the moving average output signal over the predetermined period after the symbol start timing of the receiver device. Specifically, the areas SL5 and SR5 are each derived as the sum total of L sample values of the moving average output signal. In the figures, 2L denotes an error detection range.
Where the symbol start timing generated within the receiver device coincides with the peak of the moving average output signal (symbol start timing of the received signal) as shown in FIG. 8A, the difference between the areas SL5 and SR5 is zero (“0”), so that no error is detected.
On the other hand, where the symbol start timing generated within the receiver device is earlier than that of the received signal as shown in FIG. 8B, the difference (SL5−SR5) between the areas SL5 and SR5 takes a negative value. Conversely, where the symbol start timing generated within the receiver device is later than that of the received signal as shown in FIG. 8C, the difference (SL5−SR5) between the areas SL5 and SR5 takes a positive value. In the conventional receiver device, the sampling frequency is corrected on the basis of the difference between the areas SL5 and SR5, to thereby correct the timing error of the receiver device.
As such conventional devices configured to obtain the area difference, a receiver device has been known in which an FIR (Finite Impulse Response) filter comprising 2L one-clock delayers is used to obtain the area difference.
However, in the conventional receiver devices wherein the symbol timing error of the receiver device is corrected by using the areas calculated based on the received signal, a large number of adders are required to calculate the area difference, resulting in enlargement of the scale of circuitry.
For example, in the case of the receiver device using the FIR filter comprising 2L one-clock delayers, usually (2L−1) adders are required, giving rise to a problem that widening the error detection range leads to further enlargement in the scale of circuitry.
Also, in the conventional receiver devices wherein the symbol timing error of the receiver device is corrected by using the areas calculated based on the received signal, the detected error signal undergoes variations as the input level of the received signal varies, adversely affecting the stability and convergence of the control system.